http://www.google.com/flights/

or from the US http://google.com/flights

via
http://money.cnn.com/2011/09/13/technology/google_flight_search/

and
http://insidesearch.blogspot.com/2011/05/plan-your-next-trip-with-new-flight.html

Ooops
http://insidesearch.blogspot.com/2011/09/early-look-at-our-flight-search-feature.html

Hmm, I doubt this will become available outside the US, a pity though, cause it looks pretty solid.